1. Field of the Invention
The present invention relates to a laser driving device, an optical scanning device, an image forming apparatus, and a laser driving method, and more particularly relates to a laser driving device that drives a semiconductor laser, an optical scanning device that includes the laser driving device, an image forming apparatus that includes the optical scanning device, and a laser driving method for driving a semiconductor laser.
2. Description of the Related Art
Methods for driving a semiconductor laser are generally classified into a zero bias method and a bias method. The zero bias method is a method in which a bias current of a semiconductor laser is set to zero and a current corresponding to an input signal is supplied to the semiconductor laser so as to emit light. The bias method is a method in which an oscillation threshold current of a semiconductor laser is set to a bias current that is constantly supplied, and a difference between a current corresponding to an input signal and the bias current is added to the semiconductor laser so as to emit light.
In an image forming field typified by laser printers and digital copying machines, high speed processing is rapidly developing. Also, in an optical communications field, a communication speed markedly increases to a higher speed.
When a semiconductor laser having a large oscillation threshold current is driven by the zero bias method, a light-emitting delay occurs because it takes a certain period of time to produce a carrier density capable of laser oscillation from time at which a driving current corresponding to an input signal is supplied to the semiconductor laser. In this case, if the semiconductor laser is forced to be turned on and off at high speed, even if a driving current corresponding to a desired lighting pulse width is supplied to the semiconductor laser, a practical lighting pulse width may be smaller than the desired pulse width.
On the other hand, when the semiconductor laser is driven by the bias method, the light-emitting delay is smaller than that of the zero bias method because a current corresponding to the oscillation threshold current is constantly supplied. In this case, however, the semiconductor laser constantly and slightly emits light (typically, 200 μW to 300 μW) due to the bias current even when no light emission is required. In optical communications, this constant light emission results in a lower extinction ratio, causing so called a background fog in an image forming apparatus. A light-emitting amount caused by the bias current is also called as an “offset light-emitting amount”.
Accordingly, in the optical communications field, there have been proposals that the oscillation threshold current is supplied just before starting light emission. For example, refer to Japanese Patent Application Laid-open No. H4-283978 and Japanese Patent Application Laid-open No. H9-083050.
Recently, image forming apparatuses using a red laser having an oscillation wavelength of the 650-nm range or an ultra violet laser having an oscillation wavelength of the 400-nm range have been gradually in practical use for achieving higher resolution. Such semiconductor lasers have a characteristic that a time period taken for producing a carrier density capable of laser oscillation is longer than those of the conventional lasers having oscillation wavelengths of the 1.3-μm range, the 1.5-μm range, and the 780-nm range.
When a low density image is to be expressed by shortening a pulse width (e.g., less than a few nanoseconds), a light-emitting output does not reach a peak intensity of a light spot, so that there is a problem that the image is displayed with a density lower than the desired density and therefore the correct density cannot be expressed. In order to solve the problems, various proposals have been made.
For example, Japanese Patent Application Laid-open No. H5-328071 discloses a static latent image forming apparatus that superimposes a differentiated waveform (differentiated pulse) of an element driving signal that is applied to a light-emitting element on the element driving signal.
For another example, Japanese Patent No. 3466599 discloses a semiconductor laser driving circuit that drives a semiconductor laser with a summation current of four currents: a bias current, a driving current, a threshold current, and a driving auxiliary current.
The static latent image forming apparatus disclosed in Japanese Patent Application Laid-open No. H5-328071, however, has problems in that the semiconductor laser may be put at increased risk of destruction because the device cannot control a peak value of the differentiated pulse, and an ultra low density in an initial stage may be corrected but it is not always true that a subsequent tone expression is linearly increased because a time period taken for superimposing the differentiated pulse on the element driving signal depends on the differentiated waveform.
The semiconductor laser driving circuit disclosed in Japanese Patent No. 3466599 can form a light waveform as a nearly ideal square waveform, but a so-called “pulse width reduction” may occur in which a pulse width of a light waveform becomes smaller than a pulse width of an input signal, depending on setting values of the bias current and the oscillation threshold current.
As another method, it has been also proposed that the rising characteristic of a light output is improved by overshooting a light waveform at timing at which a semiconductor laser starts to emit light. However, the overshooting sometimes causes the semiconductor laser to emit light with a light amount exceeding a predetermined light amount (e.g., a rated light amount) depending on amplitude of the overshooting, resulting in the occurrence of problems of deterioration and a short life span of the semiconductor laser, for example.